1. Field of the Invention
This invention relates generally to the field of seismic exploration and more particularly to transportable apparatus for injecting an acoustic pulse into the earth.
2. Description of the Prior Art
The invention to be described constitutes an improvement in transportable seismic sources of the type generally disclosed and illustrated in U.S. Pat. No. 4,402,381 entitled "Vehicle Mounting and Deployment Arrangement for Seismic Source," issued Sept. 6, 1983, the contents of which are hereby incorporated by reference, and in copending commonly assigned U.S. patent application Ser. No. 06/846,393 entitled "Baseplate Locator for Seismic Source," filed Mar. 31, 1986 in the name of Tom P. Airhart and Andrew B. Woodrow. As described in U.S. Pat. No. 4,402,381, an impact mass suspended within the generally upright, open bottom cylindrical firing tube of a vehicle-mounted actuator is propelled downwardly under gas pressure to impact a wide area baseplate positionable on the surface of the earth in alignment with the bore of the tube. The baseplate couples the kinetic energy of the moving mass into the earth to generate a seismic wave of acoustic frequency. According to the patent, when the baseplate is deployed at a particular location, its alignment is maintained by various means including, among other features, a plurality of vertical lift cylinders operated like jacks extending upwardly between the upper baseplate surface and the actuator. The weight of the vehicle and associated apparatus carried thereon keeps the baseplate from shifting position under repeated heavy blows from the impact mass. The above-referenced patent application concerns an improvement in the baseplate alignment means taught in U.S. Pat. No. 4,402,381 in order to achieve superior results on uneven terrain. According to such application, lift means such as previously described in the -381 patent are provided together with elastic shock isolators through which vertical forces are transmitted between the vehicle chassis and the baseplate. These isolators not only cushion the system against vertically-directed shocks but, also, provide restorative forces in the event lateral misalignment develops between the vehicle and the baseplate.
A baseplate such as described above is designed to function best when positioned on relatively soft, and thus somewhat compressible, ground or soil. Under the weight of the transport vehicle, such a baseplate tends to smooth out surface irregularities and thus establish good overall contact between the baseplate surface and the underlying earth medium. A large surface baseplate is generally preferred under these conditions because it lowers the loading per unit area under impact. This in turn makes it possible to deliver more energy to the baseplate without increasing inefficient plastic deformation of the earth to an undesirable degree. However, field tests of this type of source indicate that when a seismic line includes surfaces consisting at least intermittently of hard rock, either exposed or only lightly covered by clumps of vegetation, a large area baseplate encounters problems. One reason appears to be that upon deployment from the transport vehicle, the baseplate "bridges over" irregularities in the rock surface which are substantially immune to smoothing even under great weight. This concentrates stress unevenly over the baseplate surface, leading to strain and cracking under repeated blows. It also makes it more difficult in such environment to generate seismic waves of reliable and repeatable energy content, thus interfering with the data quality obtainable and complicating the task of seismic interpretation.